The present invention relates to an amplifier output stage comprising a first and a second transistor each provided with a bias terminal, a reference terminal and a transfer terminal, said transistors having their principal current paths arranged between a first and a second power supply terminal, a junction point between said current paths constituting an output terminal of the stage, the bias terminal of the first transistor being connected to an output of the amplifier, the bias terminal of the second transistor being connected to an input of the amplifier via a bias circuit.
Such output stages are commonly used in integrated circuits. They have the advantage of realizing an amplification of a signal present at the output of the amplifier at the expense of a low energy dissipation. The known output circuit has, however, a major drawback, which appears when the second transistor is in the saturated state. In this state, the second transistor features a parasitic transistor of a conductivity type which is opposed to that of the second transistor, the bias terminal, the reference terminal and the transfer terminal of the parasitic transistor being constituted by the transfer terminal, the bias terminal and the substrate, respectively, of the second transistor. When the second transistor is saturated, it receives a bias current at its bias terminal which is directed to the substrate via the main current path of the parasitic transistor. If the bias terminal of the second transistor has a low impedance, nothing limits the bias current value, which is precisely the case for the known output stages, in which the bias terminal of the second transistor is connected to the input of the amplifier whose equivalent impedance is necessarily low. This connection is realized either directly or via a bias circuit which may comprise a diode connected to a power supply terminal via the main current path of a transistor whose bias terminal is connected to the input of the amplifier, but the bias circuit does not in any case have a sufficient impedance to limit the bias current of the second transistor. The value of said bias current thus is not limited, because of the low impedance of the bias terminal of the second transistor, with the result that a strong parasitic current is injected into the substrate at the risk of disturbing the operation of all components present in the integrated circuit on the surface of the substrate, and may even damage this circuit. Moreover, when the second transistor leaves its saturated state so as to resume a linear operation mode under the effect of an increase of the potential at the output terminal of the stage, a necessary evacuation of the important amount of electric charge accumulated in the base of the second transistor generates a current pulse in the collector of said transistor, as the parasitic transistor has disappeared. Now, the power supply terminals of the integrated circuit are connected by means of wires to pins present on a protection housing incorporating the integrated circuit, which pins provide the electric connections between the integrated circuit and components outside the housing. These wires have an intrinsic inductance which, when subjected to a current pulse, generates a voltage peak of a very high value which may have destructive effects, or at least disturb the potential of the power supply terminals and thereby the operation of the whole integrated circuit.